As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries, which exhibit high energy density and discharge voltage, has been underway and such lithium secondary batteries are commercially available and widely used. Such lithium secondary batteries are the most widely used due to excellent electrode lifespan and high high-speed charge and discharge efficiency.
Generally, as cathode active materials for lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, and the like and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
Among cathode active materials, LiCoO2 is widely used due to excellent overall physical properties such as excellent cycle properties, and the like. However, LiCoO2 is low in safety and expensive due to resource limitations of cobalt as a raw material. Lithium nickel based oxides such as LiNiO2 are cheaper than LiCoO2 and exhibit high discharge capacity when charged to a voltage of 4.25 V. However, lithium nickel based oxides have problems such as high production cost, swelling due to gas generation in batteries, low chemical stability, high pH and the like.
In addition, lithium manganese oxides, such as LiMnO2, LiMn2O4, and the like, are advantageous in that they contain Mn, which is an abundant and environmentally friendly raw material. In particular, among the lithium manganese oxides, LiMn2O4 has advantages such as a relatively cheap price, high output and the like. On the other hand, LiMn2O4 has lower energy density, when compared with LiCoO2 and three component-based active materials.
To overcome these drawbacks, some Mn of LiMn2O4 is substituted with Ni and thereby a spinel material having a composition of Li1+aNixMn2−xO4−z(0≤a≤0.1, 0.4≤x≤0.5, and 0≤z≤0.1) has a higher potential (approximately 4.7 V) than original operating potential (approximately 4 V), and, accordingly, the spinel material is well suited to use as a cathode active material of medium and large lithium secondary batteries used in EVs requiring high energy and high-output performance. However, due to high charge and discharge voltage potential, there are a variety of problems, which must be solved, such as reduced battery performance caused by Mn dissolution of the cathode active material and side reaction of an electrolyte.
Therefore, there is a need to develop an excellent cathode active material having excellent high-speed charging characteristics not while causing the problems.